If you have a plugin A which optionally depends on plugin B.

You start with just plugin B 1.0 installed in your system.

Install plugin A 1.0 (which depends optionally on plugin B 1.1) and Jenkins does not upgrade plugin B (it should, and if the dependency was non-optional it will)

Similarly if you upgrade plugin A to 1.1 (which now optionally depends on plugin B 2.0) again Jenkins does not upgrade plugin B (even though for a non-optional plugin it will)